MEMS pressure sensors are typically formed of a flexible membrane suspended over a hollow space or gap that forms a sensor volume in a substrate. The flexible membrane is deflected based on the pressure differential of the gases on each side of the membrane. Sensing elements are connected to the membrane that are configured to detect the amount of deflection which can then be correlated to a pressure value for the pressure of the gas on the side of the membrane opposite the sensor volume. Sensing elements based on piezoresistive technology and capacitive sensing technology are typically used. An evaluation circuit, such an application specific integrated circuit (ASIC) is typically provided in the same package with the pressure sensor for evaluating the output of the sensing elements.
Humidity sensors are widely used in various fields to measure the amount of water vapor present in the air of a particular environment. Humidity sensors typically include a pair of electrodes separated by a dielectric material. The dielectric layer is formed of a material, such as polymer that is configured to absorb and retain water molecules at concentrations that are proportional to the ambient humidity. The water molecules alter the dielectric constant of the polymer resulting in a change in capacitance between the two electrodes. Humidity can therefore be determined by measuring the capacitance between the two electrodes and correlating the measured capacitance to a corresponding humidity value.
Sensor housings are usually provided with various openings to media entry for the sensors. The openings allow gas exchange between the interior volume of the sensor housing and the exterior atmosphere. The gas exchange allows the interior volume of the sensor housing to be at or near the ambient pressure and humidity. However, the semiconductor components of some devices are sensitive to the electromagnetic radiation, particularly infrared radiation. For example, piezoresistors can generate a photocurrent when light is incident upon the resistor. The photocurrent produces a voltage drop across the piezoresistor which can corrupt the measurement signal output by the sensor.
Therefore, care must be taken when media enters that the direct incidence of light on the chip is minimized, for example by as small an opening in the housing as possible. This requirement is in direct contrast to other sensor devices which benefit from having larger media openings and are not susceptible to incident light. For example, humidity sensors based on the capacitive measurement principle typically have large openings in the housing positioned directly above the sensor chip for rapid response times.
The competing requirements of smaller openings and limited light exposure for sensor chips, such as pressure sensors and ASICs, and larger openings with no limit on light exposure for sensor devices, such as humidity sensors, has made it difficult to combine such devices into the same package in a manner that enables smaller package sizes to be fabricated in a cost-effective manner.